Keeping large numbers of interdependent items consistent with each other can be a costly process. One reason is that when items change, every item in a potentially enormous set must be inspected to see if it has dependencies on any changed items. For example, if a first inspected item depends on a second changed item, then the first inspected item must be updated to make the members of the entire set of items consistent with each other once again. In addition, all items in the set that are affected by the first changed and the updated item must also be inspected to see if they depend on the updated item. The cost of updating all affected items in the set can be very high—a single small change in one item out of tens of thousands of items can trigger many hours of computer activity, including routinely checking members of the set that are not even affected by the change. Therefore it would be desirable to have a more efficient way of achieving consistency among sets of items.